It is known to provide electrically conductive security threads in currency such as U.S.currency. These security threads can be detected, using sensors which operate, based on changes in capacitance due to the presence of a security thread. The electrical conductivity of the security threads can be continuous or can be segmented.
U. S. Pat. No. 5,419,424 discloses a device for sensing of the security threads in a document. This patent discloses a structure which has a host of sensors and uses horizontal and vertically oriented electrodes in combination with a horizontally disposed feed electrode for distinguishing between security threads having discrete segments along the length thereof and a conductive line, such as a pencil line on a document.
As can be appreciated, sensing of security threads is used in combination with other sensing and evaluation techniques for collectively determining whether a particular document is authentic. Typically, the document is moved along a predetermined path and is moved past fixed sensors. These sensors provide input which is evaluated to provide a prediction whether the document is authentic as bill passes thereby. This evaluation and prediction occurs quickly as the consumer is typically waiting for the results, i.e. credit towards a purchase, etc.
In sensing currency, it can be appreciated that the condition of the currency can greatly vary from a relatively new crisp paper bill, to one which is quite worn and may have a series of creases or folds therein. The bill as it passes along the path, is normally controlled in a guide arrangement, however, there is some movement of the currency within the guide from the guide centerline, and thus the bill can wobble within the guide. This wobble can dramatically effect a capacitance sensor, which is relatively sensitive to changes in the separation distance between the bill and the sensor. Most capacitance sensors require the sensor to almost be in contact with the currency and this can cause the currency to jam in the validator. Thus with low separation spacing, the quality of the signal from the capacitance sensor improves, however there is a significant service and reliability problem, caused by jamming of bills in the validator. Furthermore, the wobble of the paper currency, as it passes through the validator, can also rapidly change the separation distance and the signals from the capacitance sensor. The signal from the capacitance sensor is expected to increase and decrease, however it is difficult to know whether these changes are caused by wobble or changing location of a security thread as it moves past a sensor.
In prior art devices there has been the tendency to decrease the separation distance between the sensor and the bill to improve the strength of the signal but this has not proven entirely satisfactory.
The present invention overcomes a number of these problems.